It is known to recirculate a portion of internal combustion engine exhaust gas to the engine fresh air intake to contribute to a reduction in the undesirable engine emission element of oxides of nitrogen NOx. Generally, exhaust gas recirculation EGR reduces the availability of oxygen during combustion in the engine, which is needed for NOx production, and increases the capacity of the engine intake air charge to absorb heat, holding combustion temperature down to levels inhibiting NOx production. The capacity of conventional internal combustion engine emission control strategies to inhibit emission of NOx corresponds on the integrity of the EGR system. The harsh environment in which the EGR system necessarily must operate, characterized by high temperatures and high levels of contaminants, contributes to a significant potential for EGR system faults, the more severe of which include restriction of EGR passageways to the engine fresh air intake. Such restriction impedes the efficiency of the EGR system in recirculating the exhaust gas to the engine fresh air intake, leading to increased emission of undesirable NOx.
Accordingly, it would be beneficial to provide for diagnosis of EGR system faults, such as may result in restriction in EGR passageways, so that timely repair or replacement may be made, for effective engine emissions reduction. To avoid unnecessary repair or replacement, a high degree of confidence should be associated with any fault that is diagnosed. The cost of the diagnostic, including any associated hardware and software, should be minimized.